U.S. patent number 8,488,923 [Application Number 12/749,196] was granted by the patent office on 2013-07-16 for multimode optical coupler interfaces.
This patent grant is currently assigned to Intel Corporation. The grantee listed for this patent is Yun-Chung Na, Tao Yin. Invention is credited to Yun-Chung Na, Tao Yin.
United States Patent |
8,488,923 |
Na , et al. |
July 16, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
Multimode optical coupler interfaces
Abstract
Optical interfaces that may be employed between large-core
optical fibers and chip-scale optoelectronic devices. Described
herein are couplers that improve the tolerance of misalignment when
a single mode (SM) fiber is used as waveguide input. This enables
the possibility of passive/automatic alignment and therefore
reduces the production cost. The coupler also serves as a spot-size
converter that reduces the spot size and is suitable for
applications where a waveguide mode with small cross-section area
is of particular importance. One such example can be a
waveguide-based SiGe or III-V semiconductor photodetector in which
the vertical size of its waveguide mode should be as small as few
microns.
Inventors: |
Na; Yun-Chung (Palo Alto,
CA), Yin; Tao (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Na; Yun-Chung
Yin; Tao |
Palo Alto
San Jose |
CA
CA |
US
US |
|
|
Assignee: |
Intel Corporation (Santa Clara,
CA)
|
Family
ID: |
44656585 |
Appl.
No.: |
12/749,196 |
Filed: |
March 29, 2010 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20110235968 A1 |
Sep 29, 2011 |
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Current U.S.
Class: |
385/28;
385/14 |
Current CPC
Class: |
G02B
6/305 (20130101); G02B 6/1228 (20130101); G02B
6/2808 (20130101); G02B 6/26 (20130101) |
Current International
Class: |
G02B
6/26 (20060101) |
Field of
Search: |
;385/28,14 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Written Opinion and Int'l Search Report for Int'l Patent
Application No. PCT/US2011/030358 mailed Jan. 2, 2012, 10 pgs.
cited by applicant .
Thurston, R.N., et al., "Two-dimensional control of mode size in
optical channel waveguides by lateral channel tapering", Optics
Letter, vol. 16, No. 5, Mar. 1, 1991, 306-308 cited by
applicant.
|
Primary Examiner: Peng; Charlie
Assistant Examiner: Radkowski; Peter
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor &
Zafman LLP
Claims
What is claimed is:
1. An multi-mode optical coupler comprising: an input multi-mode
waveguide to receive an optical signal from a single mode optical
fiber; one or more single mode output waveguides optically coupled
with the input multi-mode waveguide to form a single mode waveguide
array, the one or more single mode output waveguides to receive the
excited multiple modes caused by fiber misalignment, wherein the
widths of the single mode output waveguides are adiabatically
tapered in an increasing fashion to pull dispersions of the single
mode waveguides toward a low frequency side and sweep the input
frequencies to transfer optical power in the multimode waveguide
into the single mode waveguide array adiabatically, wherein optical
couplers to couple the one or more single mode output waveguides
with the input multi-mode waveguide possess horizontal mirror
symmetry to categorize the waveguide modes into either even or odd
parity and the multi-mode waveguide can only interact with the
single-mode waveguide array among modes with the same polarity to
cause a subset of normal-mode splittings.
2. The multi-mode optical coupler of claim 1 wherein the one or
more single mode output waveguides is embedded within or on top of
the input multi-mode waveguide.
3. The multi-mode optical coupler of claim 1 wherein the one or
more single mode output waveguides comprise single mode waveguides
having adiabatically tapered widths.
4. The multi-mode optical coupler of claim 1 wherein the multi-mode
input waveguide is at least ten times thicker than the one or more
single mode output waveguides.
5. The multi-mode optical coupler of claim 1 wherein the multi-mode
input waveguide comprises a silicon (Si) optical waveguide.
6. The multi-mode optical coupler of claim 1 wherein the multi-mode
input waveguide comprises an aluminum gallium arsenide (AlGaAs)
waveguide.
7. The multi-mode optical coupler of claim 1 wherein the one or
more single mode output waveguides comprise aluminum arsenide
(AlAs) optical waveguides.
8. The multi-mode optical coupler of claim 1 wherein the one or
more single mode output waveguides comprise silicon germanium
(SiGe) optical waveguides.
9. The multi-mode optical coupler of claim 8 wherein the one or
more SiGe single mode output waveguides have a composition of
Si.sub.xGe.sub.1-x.
10. An optical system comprising: a single mode optical fiber to
transmit an optical signal; an input multi-mode waveguide to
receive the optical signal from the single mode optical fiber; one
or more single mode output waveguides having adiabatically tapered
widths optically coupled with the input multi-mode waveguide, the
one or more single mode output waveguides to receive the excited
multiple modes caused by fiber misalignment, wherein the widths of
the single mode output waveguides are adiabatically tapered in an
increasing fashion to pull dispersions of the single mode
waveguides toward a low frequency side and sweep the input
frequencies to transfer optical power in the multi-mode waveguide
into the single mode waveguide array adiabatically, wherein optical
couplers to couple the one or more single mode output waveguides
with the input multi-mode waveguide possess horizontal mirror
symmetry to categorize the waveguide modes into either even or odd
parity and the multi-mode waveguide can only interact with the
single-mode waveguide array among modes with the same polarity to
cause a subset of normal-mode splittings.
11. The optical system of claim 10 wherein the one or more single
mode output waveguides is embedded within or on top of the input
multi-mode waveguide.
12. The optical system of claim 10 wherein the multi-mode input
waveguide is at least ten times thicker than the one or more single
mode output waveguides.
13. The optical system of claim 10 further comprising one or more
optoelectronic devices optically coupled with the one or more
output waveguides.
14. The optical system of claim 10 wherein the multi-mode input
waveguide comprises a silicon (Si) optical waveguide.
15. The optical system of claim 10 wherein the multi-mode input
waveguide comprises an aluminum gallium arsenide (AlGaAs)
waveguide.
16. The optical system of claim 10 wherein the one or more single
mode output waveguides comprise aluminum arsenide (AlAs) optical
waveguides.
17. The optical system of claim 10 wherein the one or more single
mode output waveguides comprise silicon germanium (SiGe) optical
waveguides.
18. The optical system of claim 17 wherein the one or more SiGe
single mode output waveguides have a composition of
Si.sub.xGe.sub.1-x.
19. A method for manufacturing a multi-mode optical coupler, the
method comprising: creating a multi-mode silicon (Si) strip
waveguide on a silicon dioxide (SiO2) substrate; and creating a
plurality of silicon germanium (SiGe) single mode output waveguides
on a face of the Si strip waveguide opposite the SiO2 substrate,
wherein the output waveguides have adiabatically tapered widths,
wherein the widths of the single mode output waveguides are
adiabatically tapered in an increasing fashion to pull dispersions
of the single mode waveguides toward a low frequency side and sweep
the input frequencies to transfer optical power in the multi-mode
Si strip waveguide into the single mode waveguide array
adiabatically, wherein optical couplers to couple the one or more
single mode output waveguides with the input multi-mode waveguide
possess horizontal mirror symmetry to categorize the waveguide
modes into either even or odd parity and the multi-mode waveguide
can only interact with the single-mode waveguide array among modes
with the same polarity to cause a subset of normal-mode
splittings.
20. The method of claim 19 wherein the multi-mode input waveguide
is at least ten times thicker than the output waveguides.
21. The method of claim 19 wherein the SiGe output waveguides have
a composition of Si.sub.xGe.sub.1-x.
22. A method for manufacturing a multi-mode optical coupler, the
method comprising: creating a multi-mode silicon (Si) strip
waveguide on a silicon dioxide (SiO2) substrate; and creating a
single silicon germanium (SiGe) output waveguide on a face of the
Si strip waveguide opposite the SiO2 substrate, wherein the output
waveguides have adiabatically tapered widths, wherein the widths of
the single mode output waveguides are adiabatically tapered in an
increasing fashion to pull dispersions of the single mode
waveguides toward a low frequency side and sweep the input
frequencies to transfer optical power in the multi-mode Si strip
waveguide into the single mode waveguide array adiabatically,
wherein optical couplers to couple the one or more single mode
output waveguides with the input multi-mode waveguide possess
horizontal mirror symmetry to categorize the waveguide modes into
either even or odd parity and the multi-mode waveguide can only
interact with the single-mode waveguide array among modes with the
same polarity to cause a subset of normal-mode splittings.
23. The method of claim 22 wherein the multi-mode input waveguide
is at least ten times thicker than the output waveguide.
24. The method of claim 22 wherein the SiGe output waveguides have
a composition of Si.sub.xGe.sub.1-x.
Description
TECHNICAL FIELD
Embodiments of the invention relate to optical couplers. More
particularly, embodiments of the invention relate to multimode
optical coupler interfaces for interfacing, for example, optical
fibers and photonic integrated circuits.
BACKGROUND
Coupling light from fiber to waveguide with high efficiency is
crucial in the development of integrated photonics. In particular,
single mode (SM) fiber to waveguide coupling without active/manual
alignment is very challenging because of the small spot size
involved.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are illustrated by way of example, and
not by way of limitation, in the figures of the accompanying
drawings in which like reference numerals refer to similar
elements.
FIG. 1 illustrates one embodiment of an optical coupler having an
output arrayed waveguide.
FIG. 2 illustrates one embodiment of an optical coupler having a
single output waveguide.
FIG. 3 is a mode profile for an example embodiment in which a
single mode fiber (NA.about.0.14) is coupled with a 10 .mu.m by 10
.mu.m silicon-on-insulator (SOI) strip waveguide.
FIG. 4 is a dispersion diagram that illustrates the operation of
the optical couplers described herein.
FIG. 5 provides a result from a simulation of one embodiment of an
optical coupler as described herein.
FIG. 6 is a block diagram of one embodiment of an optical system
utilizing an optical coupler as described herein.
DETAILED DESCRIPTION
In the following description, numerous specific details are set
forth. However, embodiments of the invention may be practiced
without these specific details. In other instances, well-known
circuits, structures and techniques have not been shown in detail
in order not to obscure the understanding of this description.
Described herein are optical interfaces that may be employed
between large-core optical fibers and chip-scale optoelectronic
devices. Photons can be coupled efficiently from one side to
another in a fully coherent way. Described herein are couplers that
improve the tolerance of misalignment when a single mode (SM) fiber
is used as waveguide input. This enables the possibility of
passive/automatic alignment and therefore reduces the production
cost. The coupler also serves as a spot-size converter that reduces
the spot size and is suitable for applications where a waveguide
mode with small cross-section area is of particular importance. One
such example can be a waveguide-based SiGe or III-V semiconductor
photodetector in which the vertical size of its waveguide mode
should be as small as few microns.
The optical couplers may be used for interfacing SM light source
and chip-scale photonic devices. In one embodiment, components
include 1) relatively large multimode (MM) waveguide, 2) relatively
small SM waveguide array/MM slab waveguide, and 3) inverted-taper
structure. The description that follows assumes that the material
absorption loss at the wavelength of interest is negligible, which
is true for Silicon (Si) and Silicon Germanium (SiGe) with small
Germanium (Ge) composition at telecom wavelengths such as 1.31
.mu.m and 1.55 .mu.m.
FIG. 1 illustrates one embodiment of an optical coupler having an
output arrayed waveguide. While the example of FIG. 1 demonstrates
an output arrayed waveguide sitting on top of an input waveguide,
in alternate embodiments the output waveguide can also be embedded
in the input waveguide which may further improve coupling
strength.
The optical coupler is created or assembled on substrate 100. In
one embodiment, substrate 100 is a Silicon Dioxide (SiO.sub.2)
substrate. In alternate embodiments, other substrates may be used,
for example, gallium arsenide (GaAs). The optical coupler includes
input waveguide 120 on substrate 100 to receive an optical signal
from an optical fiber (not illustrated in FIG. 1). In one
embodiment, input waveguide 120 is created from Silicon (Si). In
alternate embodiments, other materials can be used, for example,
aluminum gallium arsenide (AlGaAs).
The optical coupler further includes output arrayed waveguide 140
that is created or placed on top of input waveguide 120. Specific
sizes, relationships, and other design considerations are discussed
in greater detail below. In one embodiment, output arrayed
waveguide 140 is created from Silicon Germanium
(S.sub.xGe.sub.1-x). In alternate embodiments, other materials can
be used, for example, aluminum arsenide (AlAs).
In one embodiment, the input port of input waveguide 120 of the
optical coupler is designed to be a MM waveguide with adequate
cross-section area (e.g., .about.10 .mu.m.times.10 .mu.m) to
interface an external SM fiber. This allows the coupling from the
SM fiber into the optical coupler with negligible loss.
In one embodiment, output arrayed waveguide 140 is an array of SM
waveguides with small cross-section area (e.g., .about.1
.mu.m.times.1 .mu.m) sits on top of MM input waveguide 120. In one
embodiment, the widths of the individual waveguides of output
arrayed waveguide 140 are tapered adiabatically in an increasing
fashion. The number of the SM waveguides depends on the number of
modes in the MM waveguide to be converted. In the example of FIG.
1, output arrayed waveguide 140 includes five SM waveguides as an
example. Photons injected into the MM input waveguide 120 will be
extracted by SM waveguide array 140 because of coherent evanescent
coupling. The efficiency is mainly constrained by the
inverted-taper loss due to non-adiabaticity.
FIG. 2 illustrates one embodiment of an optical coupler having a
single output waveguide. While the example of FIG. 2 demonstrates
an output waveguide sitting on top of an input waveguide, in
alternate embodiments the output waveguide can also be embedded in
the input waveguide which may further improve coupling
strength.
The optical coupler is created or assembled on substrate 200. In
one embodiment, substrate 200 is a SiO.sub.2 substrate. In
alternate embodiments, other substrates may be used, for example,
gallium arsenide (GaAs). The optical coupler includes input
waveguide 220 on substrate 200 to receive an optical signal from an
optical fiber (not illustrated in FIG. 2). In one embodiment, input
waveguide 220 is created from Silicon. In alternate embodiments,
other materials can be used, for example, aluminum gallium arsenide
(AlGaAs).
The optical coupler further includes single output waveguide 240
that is created or placed on top of input waveguide 220. Specific
size, relationships, and other design considerations are discussed
in greater detail below. In one embodiment, single output waveguide
240 is created from Silicon Germanium (S.sub.xGe.sub.1-x). In
alternate embodiments, other materials can be used, for example,
aluminum arsenide (AlAs).
In one embodiment, the input port of input waveguide 220 of the
optical coupler is designed to be a MM waveguide with adequate
cross-section area (e.g., .about.10 .mu.m.times.10 .mu.m) to
interface an external SM fiber. This allows the coupling from the
SM fiber into the optical coupler with negligible loss.
In one embodiment, single output waveguide 240 is a SM waveguide
with small thickness (e.g., .about.1 .mu.m) sits on top of MM input
waveguide 220. In one embodiment, the width of single output
waveguide 240 is tapered adiabatically in an increasing fashion.
Photons injected into the MM input waveguide 220 will be extracted
by SM output waveguide 240 because of coherent evanescent coupling.
The efficiency is mainly constrained by the inverted-taper loss due
to non-adiabaticity.
An important benefit of the optical couplers describe herein is the
tolerance of misalignment. Assuming a standard SM fiber with NA
.about.0.14 is coupled to a 10 .mu.m.times.10 .mu.m SOI
(Silicon-on-Insulator) strip waveguide, the maximum number of modes
can be excited is about three. Profiles for the three modes are
provided in FIG. 3. Because the optical couplers described herein
can couple not only the fundamental mode but also higher-order
modes into the small waveguide array, it offers a larger tolerance
of misalignment compared to the conventional designs where only the
fundamental mode can be coupled.
The tolerance of fiber center-to-waveguide center offset is
calculated as .about.16 .mu.m.sup.2 for the optical coupler
described herein as compared to .about.7.8 .mu.m.sup.2 for the
conventional design. Such an improvement enables the possibility of
passive/automatic alignment and therefore reduces the optical
coupler production cost. Realistically, the improvement can be even
larger because in the example provided above, the coupling
efficiency of fundamental mode was artificially maximized by an
appropriate spot size.
FIG. 4 is a dispersion diagram that illustrates the operation of
the optical couplers described herein. The dispersion diagram of
FIG. 4 illustrates the wavevector-frequency relation for the
optical couplers. The couplers possess horizontal mirror symmetry
so that the waveguide modes can be categorized into either even or
odd parity. For the MM waveguide (SM waveguide array), the
dispersions of its lowest three modes are indicated by the solid
lines with larger slope (smaller slope). Parities are labeled
correspondingly.
The MM waveguide can only interact with the SM waveguide array
among modes with the same parity, which causes five (instead of
nine) normal-mode splittings as indicated by the dashed lines.
Assume the input wavevectors/frequencies are on the lower end of MM
waveguide dispersions, which is determined by the SM fiber
excitation condition. By adiabatically tapering the widths of SM
waveguides in an increasing fashion, the dispersions of SM
waveguide array can be "pulled" toward the large wavevector/low
frequency side, and eventually sweep the input
wavevectors/frequencies. This would transfer the optical power in
the MM waveguide into the SM waveguide array adiabatically.
Note that the number of SM waveguides used can be deducted from the
dispersions. As an example where (00), (10), (01), (11), (20),
(02), (21), (12), (30) are the lowest nine modes in the MM
waveguide, one needs nine SM waveguides to capture all of them due
to parity selection.
FIG. 5 provides a result from a simulation of one embodiment of an
optical coupler as described herein. The simulation corresponding
to FIG. 5 is based on three small SiGe waveguides (1 .mu.m.times.1
.mu.m; n=3.6) with inter-distance of 0.5 .mu.m placed on top of a
large Si waveguide (10 .mu.m.times.8 .mu.m; n=3.5), and surrounded
by SiO2 (n=1.447). Note that for 0.5 .mu.m (or smaller)
inter-distance, the three SiGe waveguides are coupled to each other
so that the degenerate modes of theirs can form definite
parities.
The dispersions of the input port are the same as for FIG. 3, where
the lowest three modes of the silicon waveguide at wavelength of
1.3 .mu.m are plotted. After an adiabatic transition by increasing
the widths of three SiGe waveguides, the dispersions of the output
port are changed as shown in FIG. 5. The mode powers are now
converted from Si waveguide to SiGe waveguides with corresponding
parities. For simplicity, the simulations here consider only TM
polarization but can be extended to TE polarization. The length of
inverted-taper is expected to be smaller than 1 mm with optimum
design.
Note that a single MM slab waveguide (as shown in FIG. 2) instead
of SM waveguide array (shown in FIG. 1) can also perform similar
functions. The dimensions of the output port will be similar but
the overall taper length can be longer for the case of MM slab
waveguide of FIG. 2.
FIG. 6 is a block diagram of one embodiment of an optical system
utilizing an optical coupler as described herein. While the example
of FIG. 6 demonstrates an output waveguide sitting on top of an
input waveguide, in alternate embodiments the output waveguide can
also be embedded in the input waveguide which may further improve
coupling strength.
Optical system 600 utilizes the optical coupler as an interface
between an optical fiber and an optoelectronic device. Optical
fiber 610 can be any type of single mode optical fiber known in the
art. Optical fiber 610 carries an optical signal from a source (not
illustrated in FIG. 6). Optical fiber 610 is optically aligned with
input waveguide 630 so that input optical signal 620 is received by
input waveguide 630.
Input waveguide 630 and output waveguide 640 together provide an
optical coupler as described in greater detail above. Output
waveguide 640 is optically aligned with optoelectronic device 660
so that output optical signal 650 is received by optoelectronic
device 660, which may be any type of optoelectronic device known in
the art. The optical couplers described herein may be used in other
situations as well. FIG. 6 is merely one example of a use of an
optical coupler.
Reference in the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. The
appearances of the phrase "in one embodiment" in various places in
the specification are not necessarily all referring to the same
embodiment.
While the invention has been described in terms of several
embodiments, those skilled in the art will recognize that the
invention is not limited to the embodiments described, but can be
practiced with modification and alteration within the spirit and
scope of the appended claims. The description is thus to be
regarded as illustrative instead of limiting.
* * * * *